1. The Field of the Invention
The present disclosure relates in general to methods of terrain correction for potential field geophysical survey data, particularly gravity and gravity gradiometry data.
2. The Related Technology
Potential field geophysical surveys are performed by measuring vector and/or tensors of a potential field, i.e., the first or second order spatial derivatives of the magnetic and/or gravity potentials. Potential field geophysical surveys are widely used for mapping subsurface geological and man-made structures in mineral, hydrocarbon, geothermal and groundwater exploration, and hydrocarbon, geothermal and groundwater resource monitoring, earth systems modeling, and tunnel and underground facility (UGF) detection.
Potential field geophysical data usually include at least one component of the gravity and/or magnetic vector and/or tensor fields. Measured potential field data contains the linear superposition of responses from both the terrain, and subsurface geological structures located beneath it.
To provide economical reconnaissance of subsurface geological structures, potential field sensors are often deployed from moving platforms such as airplanes, helicopters, airships, unattended aerial vehicles (UAV), borehole logging instruments, vessels, submarines, and vehicles.
For example, airborne potential field surveys from fixed wing aircraft typically acquire 500 line km of data each day, and airborne potential field surveys from helicopters typically acquire 200 line km of data each day. Airborne potential field surveys typically contain multiple survey lines draped over a variable terrain surface that aggregate as hundreds to thousands of line kilometers of potential field data measured every few meters and cover an area hundreds to thousands of square kilometers in size. The airborne platform's drape surface is constrained by the minimum ground clearance that the airborne platform is permitted to fly, and the maximum rate of climb or descent of the airborne platform relative to the terrain.
In practice, terrain measurements are usually embodied in Digital Elevation Models (DEM) as Easting (or Longitude), Northing (or Latitude) and Elevation constructed from the processing of (Differential) Global Position Systems ((D)GPS) and/or Radio Detection and Ranging (RADAR) and/or Laser Imaging Detection and Ranging (LIDAR) and/or Synthetic Aperture Radar (SAR) data measured from airborne platforms, or from pubic geospatial databases such as NASA's Shuttle Topography Radar Mission (SRTM).
The effect of terrain contributes to the measured potential field data. The effect of terrain actually dominates free-air gravity and gravity gradiometry data. To give an indication of the magnitude of the different responses in gravity gradiometry, a typical terrain response maybe in the order of hundreds of Eotvos (Eö), whereas the target response maybe in the order of tens of Eö.
After the potential field geophysical data have been acquired, a terrain correction needs to applied to compensate for the variations in topography (White et al., 1995, U.S. Pat. No. 5,402,340).
One method for terrain correction involves three-dimensional (3D) modeling of the potential field response in the spatial domain due to the DEM discretized using a regular or irregular grid of right rectangular prisms (“cells”) bound above by the DEM, and below by a plane that passes through the lowest elevation of the DEM. Expressions for the potential field response of each cell contain volume integrals, and these may be evaluated using analytical or numerical techniques. The cells in the grid are usually populated with a constant physical property, e.g., a constant density of 2.67 g/cc corresponding to an average upper continental crust density. In some implementations, for example for sedimentary basins, the grid is populated with a spatially variable physical property, e.g., a density analytically increasing with depth corresponding to sediment compaction. To reduce computational requirements, the cell size is increased with increasing distance from the observation point (Brewster, 2006, US Patent Application US2006/0036367 A1; Davies, 2009, US Patent Application US2009/0252372 A1; Barnes, 2009, US Patent Application US2009/0287464 A1; Barnes et al., 2010, US Patent Application US 2010/0094556 A1; Davies, 2010, US Patent Application US 2010/0206557).
Another method for terrain correction involves three-dimensional (3D) modeling of the potential field response in the Fourier (or wavenumber) domain due to the DEM discretized using a polygon bound above by the DEM, and below by a plane that passes through the lowest elevation of the DEM. Expressions for the potential field response of each cell contain Fourier transforms, and these may be evaluated using numerical techniques such as Fast Fourier Transforms (FFTs). The polygon is usually populated with a constant physical property, e.g., a constant density of 2.67 g/cc corresponding to an average upper continental crust density. In some implementations, for example for sedimentary basins, the polygon is populated with a spatially variable physical property, e.g., a density analytically increasing with depth corresponding to sediment compaction.
The accuracies of these terrain corrections are limited by numerical considerations, such as the discretization of the cells, accuracy of the volume integration, and accuracy of the Fourier transforms used. Accurate terrain corrections are computationally time consuming.
Despite improved navigation, instrumentation, and data processing techniques, there remains a need for improved terrain correction techniques for potential field geophysical data. This will be more important as next generation of potential field instruments such as very low noise (e.g., 1 Eö/√Hz) gravity gradiometers are currently being developed and tested.